expressions: functions for core apis & shared services

[ppisljar]

usage of core apis & shared services in expressions should be limited, as any function using them is no longer pure and with many api's like accessing saved objects it has other problems like reference extraction.

we should provide basic functions for:

• search: SearchSource / esaggs
• 1 search: low-level API / esdsl
• 2 index patterns: get (for esaggs)
• 1 index patterns: create
• 1 saved settings: get
• 1 saved search: get (for kibana_context)
• 2 query: kql & lucene functions (for kibana_context)
• 2 filters: esFilter function (for kibana_context)
• 2 timerange: timeRange function (for kibana_context, aggs)
• 1 saved objects: get (lower priority / nice-to-have)

for each of these items we need to:

• register a new function to the expression service
• return its own expression type so that other functions using these services in arguments can enforce that the service is accessed via the appropriate function
• add a toExpressionAst method or similar utility that can convert an instance to its AST representation using the new function, or in some cases adding utilities for serializing/deserializing the instances

related: #59960

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[ppisljar]

closing this as the majority of work has been completed and we have separate issues to track remaining work